1. Field of the Invention
The present invention relates generally to the field of helical piles. More specifically, the present invention discloses a system for field testing the bearing behavior of helical piles.
2. Statement of the Problem
Helical piles have been used for many years in a variety of applications. For example, helical piles are widely used to lift and stabilize structures, and also in new construction. In these applications, helical piles are often called upon to carry large compressive loads. Helical piles are also used to anchor structures, such as large antennas or pylons for high voltage lines, that are subject to large wind loads. Helical piles are also employed for lateral earth retention and shoring. In these fields, helical piles are often required to carry large tensile loads. For the purposes of this patent application, the terms “helical pile” and “helical pier” are used interchangeably.
Conventional helical piles have an elongated shaft with a substantially square cross-section. The shaft can either be solid or tubular. One or more helical bearing plates are permanently attached to the shaft adjacent to its lower end. The length of the shaft is fixed, as are the diameter and location of the helical plate. Examples of modular helical piering systems include the Applicant's U.S. Pat. Nos. 6,352,391, 6,682,267 7,037,045 and 6,817,810.
The helical pile is typically screwed into the ground by a power drive head that engages the upper end of the shaft and applies torque that is carried by the shaft to the helical plate. An axial force is also applied to the shaft of the helical pile. Helical piles are typically screwed into the ground to a point at which a predetermined torque limit is reached. However, it is sometimes difficult to predict the load bearing behavior of a helical pile, in tension or compression, due primarily to the unpredictable nature of soil conditions. Therefore, many jobs require field testing of the load bearing behavior of helical piles after the piles have been driven into the ground, but before subsequent phases of the job continue.
One conventional approach requires a very large weight (i.e., several tons) that is applied to the exposed head of the pile. The resulting deflections of the pile are then measured. This approach has obvious shortcomings in terms of transportation costs, space requirements and safety associated with using a large weight.
Other conventional testing techniques apply a smaller dynamic load (e.g. a small weight that impacts the head of the pile, or an explosive force) to create transient deflections in the pile that can be monitored and measured. However, these testing techniques typically require sophisticated instruments and highly-skilled technicians to interpret such data.
Another conventional approach employs a load beam held by a series of anchor piles driven into the ground. A jack is placed between the load beam and the head of the test pile to exert a test load on the test pile. The resulting reactive load is carried by the load beam and the anchor piles.
U.S. Pat. No. 7,175,368 (Stötzer) discloses a test setup for determining the bearing behavior of a displacement pile that employs a plurality of angled anchor piles to create a support structure. An hydraulic pressing device is placed between the loading device at the apex of the support structure and the test pile. However, this system does not appear to specify any particular means for measuring deflection of the test pile.
Thus, a need continues to exist in the industry for a testing system that can be readily transported and assembled in the field, and that does not require highly-skilled technicians for operation. In particular, the testing system should provide a means for directly measuring movement of the test pile under the test load.
3. Solution to the Problem
The present invention addresses these concerns by providing a testing system for helical piles that uses a set of angled anchor piles to support a head assembly with a vertical passageway aligned above the test pile. A hollow-core hydraulic jack is mounted between the upper end of the test pile and the head assembly to exert a test load on the test pile. A vertical member can be inserted through the vertical passageways of the head assembly and hollow-core hydraulic jack to allow direct measurement of the resulting movement of the test pile.
It should also be noted that the present testing system eliminates the need to transport heavy loads to the job site to test installation of helical piles. In particular, the present invention requires only components that can be readily transported to a job site and quickly assembled by one or two workmen.